So far, a speaker has been attached onto a door, rear shelf or the like of a car. With reference to FIG. 23, a description will be given herein below of one example of an installation structure of a speaker. In a door panel 104 of a car, there is made an opening 105 which is used for mounting a speaker unit 101.
At predetermined positions around this opening 105, a plurality of fitting holes 104a are formed to accept grommets 102 to be threadly engaged with small screws 103.
Accordingly, a worker aligns screw holes 101a made in the speaker unit 101 with the grommets 102 fitted in the door panel 104, and passes the screws 103 through the screw holes 101a and threadly engages the screws 103 with fitting holes 102a of the grommets 102 while maintaining the state in which the speaker unit 101 is located at the opening 105.
In addition, through such a work, the speaker unit 101 is installed at a predetermined position of the opening 105 of the door panel 104.
However, this installation structure requires the employment of the grommets 102 because consideration is given to the generation of rust in the speaker unit attaching section, which leads to an increase in number of parts. In addition, this requires a large amount of man-power for the installation of the speaker unit 101.
Accordingly, the aforesaid installation structure creates, for example, the following problems:
(1) a high manufacturing cost;
(2) a long assembling time;
(3) a requirement for precise alignment between the screw holes 101a of the speaker unit 101 and the grommets 102, which lowers the working efficiency or workability; and
(4) a need for a worker to operate while holding the speaker unit 101, the screws 103 and a tool (driver) simultaneously, which lowers the productivity.
For this reason, a rotary installation structure, which has been employed recently in various fields, has been proposed in place of such a speaker installation structure using screws.
This rotary installation structure is such that a slit having wide first slit portions and narrow second slit portions communicated with the first slit portions is made in an attached side (in the above-mentioned example, the door panel) and projections are formed on an attaching side (in the aforesaid example, the speaker unit) and the attaching side (speaker unit) is rotated in a state where the projections are inserted into the first slit portions to engage the projections, formed on the attaching side, with groove portions, made in the attached side, at the second slit portions, thereby fixedly securing the attaching side.
This construction can eliminate the need for the employment of the grommets, screws and others, which are required in the conventional technique, and permits the installation work without using a special tool. For example, Japanese Patent Laid-Open No. 2001-169374 discloses a technique in which this rotary attachment structure is applied to speaker installation.
With reference to FIG. 24, a brief description will be given hereinbelow of the technique disclosed in the aforesaid Japanese Patent Laid-Open No. 2001-169374. A perspective view of FIG. 24 illustrates a state immediately before the installation of a speaker unit 101 onto a speaker unit mounting plate 160 on a door panel 201 of a vehicle.
The speaker unit 101 includes a speaker body (main body) 111 having a resin-made speaker frame 120 and a drip-proof cover 113. Moreover, the speaker body 111 is held at a predetermined position of a generally central portion of the ring-like speaker frame 120 by bridges 120b radially formed in the interior of the speaker frame 120.
In addition, on the bridges 120b, there are integrally formed a plurality of engaging pieces 140 (upper, lower, right and left pieces; four in total) which protrude to the back side. Still additionally, one contact piece 150 is formed on the bridge 120b near the top portion of the speaker frame 120. The contact piece 150 is formed to protrude to the back side and made to have elasticity in a radial direction of the speaker frame 120. Yet additionally, on the rear surface of the speaker frame 120, a cushion 120a is put at a place which comes into contact with the speaker unit mounting plate 160.
Furthermore, a mounting opening 170 having a step is made in the speaker unit mounting plate 160, and engaging piece insertion notch portions 180a are made at positions corresponding to the engaging pieces 140 in the mounting opening 170. An opening portion of each of the engaging piece insertion notch portions 180a is connected through an inclined portion 180b to a narrow engaging piece engaging portion 180c. Still furthermore, in the mounting opening 170, a contact piece insertion notch portion 190a is made at a position corresponding to the contact piece 150, a contact piece stopping notch portion 190b is made to follow that contact piece insertion notch portion 190a. Yet furthermore, a stopping portion 190c is formed between the contact piece insertion notch portion 190a and the contact piece stopping notch portion 190b. 
Thus, in mounting the speaker unit 101 to the speaker unit mounting plate 160, in FIG. 24, the speaker unit 101 is inserted into the mounting opening 170 from the front side of the speaker unit mounting plate 160. At this time, the engaging piece 140 is inserted into the engaging piece insertion notch portion 180a, and the contact piece 150 is inserted into the contact piece insertion notch portion 190a. 
In addition, in this state, the speaker unit 101 is rotated in a direction indicated by an arrow Z. In consequence, each of the engaging pieces 140 is guided by the inclined portion 180b to be engaged with the engaging piece engaging portion 180c so that the speaker unit 101 is surely and firmly fixed to the speaker unit mounting plate 160.
Still additionally, when the speaker unit 101 is rotated in the arrow Z direction, the contact piece 150 goes from the contact piece insertion notch portion 190a through the stopping portion 190c to engage with the contact piece stopping notch portion 190b. Accordingly, even if the speaker unit 101 is rotated in the opposite direction (a direction opposite to the arrow Z direction), the contact piece 150 is brought into contact with the stopping portion 190c to inhibit the rotation of the speaker unit 101 in the opposite direction.
Yet additionally, an inner circumferential wall (flange) 121 is formed on an inner circumferential surface of the mounting opening 170 to protrude to the back side. When the speaker unit 101 is set on the speaker unit mounting plate 160, this inner circumferential wall 121 is brought into surface contact with the drip-proof cover 113 of the speaker unit 101 to enhance the installation strength of the speaker unit 101 and further to prevent the looseness at the installation. Add to it that the employment of this inner circumferential wall 121 achieves the improvement of the strength of the mounting plate 160 itself.
However, the technique disclosed in the above-mentioned Japanese Patent Laid-Open No. 2001-169374 creates the following problems.
That is, in the case of a speaker for motor vehicles, particularly for doors, the installation place is strictly limited, and the speaker 101 is required to be firmly fixed to withstand the impacts stemming from the vibration of the vehicle body or the opening and closure of the door.
For securing a high rigidity, it would be considered that the opening itself is made to have as small diameter as possible. That is, the reduction of the opening area improves the rigidity of the speaker unit mounting plate 160 accordingly.
However, since the speaker 101 requires the employment of the drip-proof cover 113 and others, limitation is imposed on the reduction of the diameter of the opening 170. Moreover, for the acoustic characteristic, it is desirable that the opening area is large, but this disagrees with it.
In addition, for enhancing the installation rigidity of the speaker 101, it is preferable that the notch portion 180a is brought close to the opening 170 to the utmost. This is because the strength of the mounting plate 160 increases as it is put closer to the inner circumferential wall (flange) 121.
However, from a processing point of view, the formation of the flange 121 makes it difficult to form the notch portion 180a in the vicinity of the flange 121.
Thus, in the case of the formation of the flange 121, the notch portion 180a is required to be located at a position remote from the opening edge in some degree and difficulty is encountered in improving the above-mentioned installation rigidity of the speaker 101.
Moreover, in the above-described conventional speaker installation structure, as a problem, working holes or the like for installation are not made particularly around the speaker unit, so it is not easy to hold the speaker. Still moreover, in mounting the speaker while rotating it, the speaker is slippy on hand, which lowers the working efficiency.
Still moreover, the above-described conventional technique requires the special formation of a rotation preventing contact piece (see reference numeral 15 in Japanese Patent Laid-Open NO. 2001-169374) for limiting the reverse rotation of the speaker to the detachment side in order to prevent the rotation of the speaker after the installation, which leads to a complicated construction.
Yet moreover, the conventional technique discloses that the cushion 120a is put between the speaker and the door panel. Although it does not disclose the material for the formation of the cushion 120a at all, a general material is a sponge (urethane foam).
However, in the case of the rotary installation structure, since the sponge 120a produces a large sliding resistance, the installation efficiency is low and a partial break occurs at rotation, which impairs the sealing performance.
On the other hand, a surface of a speaker to be installed in a car is covered with a trim material or the like, and between the speaker and the trim material, there is interposed a material such as an urethane foam or sponge, for example, disclosed in Japanese Patent Laid-Open No. HEI 11-342800.
In addition, for exhibiting a function as a speaker, sometimes, there is interposed a spacer, called a gasket or arrow rubber, which limits its height direction to prevent the contact of the rim material or the like with the cover even if a speaker cone edge vibrates. Thus, Japanese Patent Laid-Open No. 2000-36987 discloses a technique of integrally forming the cone edge and the arrow rubber.
Although the material such as a sponge can previously be adhered to the speaker side or the trim side, these encounter troublesome positioning in the case of the adhesion of the sponge, which costs much labor.
Still additionally, because of easy deformation, the urethane foam or sponge, when put to use, is required to have some degree of thickness, otherwise the interference with the trim occurs, thereby generating a rattling sound, trembling sound or the like. Yet additionally, there is a need to use a gasket or arrow rubber separately, which increases the number of parts.
The present invention has been developed with a view to eliminating the above-mentioned problems, and it is therefore an object of the invention to 1) simplify an installation structure for a speaker and further to enhance the installation rigidity of a speaker without impairing the acoustic characteristic of the speaker, 2) improve the installation work efficiency, 3) prevent the dropout or looseness of the speaker unit, 4) improve the seal performance, and 5) decrease the number of parts of the speaker.